erl_driver

C Library

erl_driver

Library Summary

API functions for an Erlang driver.

Description

An Erlang driver is a library containing a set of native driver callback functions that the Erlang Virtual Machine calls when certain events occur. There can be multiple instances of a driver, each instance is associated with an Erlang port.

Warning

Use this functionality with extreme care.

A driver callback is executed as a direct extension of the native code of the VM. Execution is not made in a safe environment. The VM cannot provide the same services as provided when executing Erlang code, such as pre-emptive scheduling or memory protection. If the driver callback function does not behave well, the whole VM will misbehave.

A driver callback that crash will crash the whole VM.

An erroneously implemented driver callback can cause a VM internal state inconsistency, which can cause a crash of the VM, or miscellaneous misbehaviors of the VM at any point after the call to the driver callback.

A driver callback doing lengthy work before returning degrades responsiveness of the VM and can cause miscellaneous strange behaviors. Such strange behaviors include, but are not limited to, extreme memory usage and bad load balancing between schedulers. Strange behaviors that can occur because of lengthy work can also vary between Erlang/OTP releases.

As from ERTS 5.5.3 the driver interface has been extended (see extended marker). The extended interface introduces version management, the possibility to pass capability flags (see driver_flags) to the runtime system at driver initialization, and some new driver API functions.

The driver calls back to the emulator, using the API functions declared in erl_driver.h. They are used for outputting data from the driver, using timers, and so on.

Each driver instance is associated with a port. Every port has a port owner process. Communication with the port is normally done through the port owner process. Most of the functions take the port handle as an argument. This identifies the driver instance. Notice that this port handle must be stored by the driver, it is not given when the driver is called from the emulator (see driver_entry).

Some of the functions take a parameter of type ErlDrvBinary, a driver binary. It is to be both allocated and freed by the caller. Using a binary directly avoids one extra copying of data.

Many of the output functions have a "header buffer", with hbuf and hlen parameters. This buffer is sent as a list before the binary (or list, depending on port mode) that is sent. This is convenient when matching on messages received from the port. (Although in the latest Erlang versions there is the binary syntax, which enables you to match on the beginning of a binary.)

In the runtime system with SMP support, drivers are locked either on driver level or port level (driver instance level). By default driver level locking will be used, that is, only one emulator thread will execute code in the driver at a time. If port level locking is used, multiple emulator threads can execute code in the driver at the same time. Only one thread at a time will call driver callbacks corresponding to the same port, though. To enable port level locking, set the ERL_DRV_FLAG_USE_PORT_LOCKINGdriver flag in the driver_entry used by the driver. When port level locking is used, the driver writer is responsible for synchronizing all accesses to data shared by the ports (driver instances).

Most drivers written before the runtime system with SMP support existed can run in the runtime system with SMP support, without being rewritten, if driver level locking is used.

Note

It is assumed that drivers do not access other drivers. If drivers access each other, they must provide their own mechanism for thread-safe synchronization. Such "inter-driver communication" is strongly discouraged.

Previously, in the runtime system without SMP support, specific driver callbacks were always called from the same thread. This is not the case in the runtime system with SMP support. Regardless of locking scheme used, calls to driver callbacks can be made from different threads. For example, two consecutive calls to exactly the same callback for exactly the same port can be made from two different threads. This is for most drivers not a problem, but it can be. Drivers that depend on all callbacks that are called in the same thread, must be rewritten before they are used in the runtime system with SMP support.

Note

Regardless of locking scheme used, calls to driver callbacks can be made from different threads.

Most functions in this API are not thread-safe, that is, they cannot be called from any thread. Functions that are not documented as thread-safe can only be called from driver callbacks or function calls descending from a driver callback call. Notice that driver callbacks can be called from different threads. This, however, is not a problem for any function in this API, as the emulator has control over these threads.

Warning

Functions not explicitly documented as thread-safe are not thread safe. Also notice that some functions are only thread-safe when used in a runtime system with SMP support.

A function not explicitly documented as thread-safe can, at some point in time, have a thread-safe implementation in the runtime system. Such an implementation can however change to a thread unsafe implementation at any time without any notice.

Only use functions explicitly documented as thread-safe from arbitrary threads.

As mentioned in the warning text at the beginning of this section, it is of vital importance that a driver callback returns relatively fast. It is difficult to give an exact maximum amount of time that a driver callback is allowed to work, but usually a well-behaving driver callback is to return within 1 millisecond. This can be achieved using different approaches. If you have full control over the code to execute in the driver callback, the best approach is to divide the work into multiple chunks of work, and trigger multiple calls to the time-out callback using zero time-outs. Function erl_drv_consume_timeslice can be useful to determine when to trigger such time-out callback calls. However, sometimes it cannot be implemented this way, for example when calling third-party libraries. In this case, you typically want to dispatch the work to another thread. Information about thread primitives is provided below.

Functionality

All functions that a driver needs to do with Erlang are performed through driver API functions. Functions exist for the following functionality:

Timer functions

Control the timer that a driver can use. The timer has the emulator call the timeout entry function after a specified time. Only one timer is available for each driver instance.

Queue handling

Every driver instance has an associated queue. This queue is a SysIOVec, which works as a buffer. It is mostly used for the driver to buffer data that is to be written to a device, it is a byte stream. If the port owner process closes the driver, and the queue is not empty, the driver is not closed. This enables the driver to flush its buffers before closing.

The queue can be manipulated from any threads if a port data lock is used. For more information, see ErlDrvPDL.

Output functions

With these functions, the driver sends data back to the emulator. The data is received as messages by the port owner process, see erlang:open_port/2. The vector function and the function taking a driver binary are faster, as they avoid copying the data buffer. There is also a fast way of sending terms from the driver, without going through the binary term format.

Failure

The driver can exit and signal errors up to Erlang. This is only for severe errors, when the driver cannot possibly keep open.

Asynchronous calls

Erlang/OTP R7B and later versions have provision for asynchronous function calls, using a thread pool provided by Erlang. There is also a select call, which can be used for asynchronous drivers.

Multi-threading

A POSIX thread like API for multi-threading is provided. The Erlang driver thread API only provides a subset of the functionality provided by the POSIX thread API. The subset provided is more or less the basic functionality needed for multi-threaded programming:

The Erlang driver thread API can be used in conjunction with the POSIX thread API on UN-ices and with the Windows native thread API on Windows. The Erlang driver thread API has the advantage of being portable, but there can exist situations where you want to use functionality from the POSIX thread API or the Windows native thread API.

The Erlang driver thread API only returns error codes when it is reasonable to recover from an error condition. If it is not reasonable to recover from an error condition, the whole runtime system is terminated. For example, if a create mutex operation fails, an error code is returned, but if a lock operation on a mutex fails, the whole runtime system is terminated.

Notice that there is no "condition variable wait with time-out" in the Erlang driver thread API. This because of issues with pthread_cond_timedwait. When the system clock suddenly is changed, it is not always guaranteed that you will wake up from the call as expected. An Erlang runtime system must be able to cope with sudden changes of the system clock. Therefore, we have omitted it from the Erlang driver thread API. In the Erlang driver case, time-outs can and are to be handled with the timer functionality of the Erlang driver API.

In order for the Erlang driver thread API to function, thread support must be enabled in the runtime system. An Erlang driver can check if thread support is enabled by use of driver_system_info. Notice that some functions in the Erlang driver API are thread-safe only when the runtime system has SMP support, also this information can be retrieved through driver_system_info. Also notice that many functions in the Erlang driver API are not thread-safe, regardless of whether SMP support is enabled or not. If a function is not documented as thread-safe, it is not thread-safe.

Note

When executing in an emulator thread, it is very important that you unlock all locks you have locked before letting the thread out of your control; otherwise you are very likely to deadlock the whole emulator.

If you need to use thread-specific data in an emulator thread, only have the thread-specific data set while the thread is under your control, and clear the thread-specific data before you let the thread out of your control.

In the future, debug functionality will probably be integrated with the Erlang driver thread API. All functions that create entities take a name argument. Currently the name argument is unused, but it will be used when the debug functionality is implemented. If you name all entities created well, the debug functionality will be able to give you better error reports.

Adding/removing drivers

A driver can add and later remove drivers.

Monitoring processes

A driver can monitor a process that does not own a port.

Version management

Version management is enabled for drivers that have set the extended_marker field of their driver_entry to ERL_DRV_EXTENDED_MARKER. erl_driver.h defines:

ERL_DRV_EXTENDED_MARKER

ERL_DRV_EXTENDED_MAJOR_VERSION, which is incremented when driver incompatible changes are made to the Erlang runtime system. Normally it suffices to recompile drivers when ERL_DRV_EXTENDED_MAJOR_VERSION has changed, but it can, under rare circumstances, mean that drivers must be slightly modified. If so, this will of course be documented.

ERL_DRV_EXTENDED_MINOR_VERSION, which is incremented when new features are added. The runtime system uses the minor version of the driver to determine what features to use.

The runtime system normally refuses to load a driver if the major versions differ, or if the major versions are equal and the minor version used by the driver is greater than the one used by the runtime system. Old drivers with lower major versions are however allowed after a bump of the major version during a transition period of two major releases. Such old drivers can, however, fail if deprecated features are used.

The emulator refuses to load a driver that does not use the extended driver interface, to allow for 64-bit capable drivers, as incompatible type changes for the callbacks output, control, and call were introduced in Erlang/OTP R15B. A driver written with the old types would compile with warnings and when called return garbage sizes to the emulator, causing it to read random memory and create huge incorrect result blobs.

Therefore it is not enough to only recompile drivers written with version management for pre R15B types; the types must be changed in the driver suggesting other rewrites, especially regarding size variables. Investigate all warnings when recompiling.

Also, the API driver functions driver_output* and driver_vec_to_buf, driver_alloc/realloc*, and the driver_* queue functions were changed to have larger length arguments and return values. This is a lesser problem, as code that passes smaller types gets them auto-converted in the calls, and as long as the driver does not handle sizes that overflow an int, all will work as before.

Rewrites for 64-Bit Driver Interface

To not update a driver and only recompile, it probably works when building for a 32-bit machine creating a false sense of security. Hopefully that will generate many important warnings. But when recompiling the same driver later on for a 64-bit machine, there will be warnings and almost certainly crashes. So it is a bad idea to postpone updating the driver and not fixing the warnings.

When recompiling with gcc, use flag -Wstrict-prototypes to get better warnings. Try to find a similar flag if you use another compiler.

The following is a checklist for rewriting a pre ERTS 5.9 driver, most important first:

Return types for driver callbacks

Rrewrite driver callback control to use return type ErlDrvSSizeT instead of int.

These changes are essential not to crash the emulator or worse cause malfunction. Without them a driver can return garbage in the high 32 bits to the emulator, causing it to build a huge result from random bytes, either crashing on memory allocation or succeeding with a random result from the driver call.

Sane compiler's calling conventions probably make these changes necessary only for a driver to handle data chunks that require 64-bit size fields (mostly larger than 2 GB, as that is what an int of 32 bits can hold). But it is possible to think of non-sane calling conventions that would make the driver callbacks mix up the arguments causing malfunction.

Note

The argument type change is from signed to unsigned. This can cause problems for, for example, loop termination conditions or error conditions if you only change the types all over the place.

Larger size field in ErlIOVec

The size field in ErlIOVec has been changed to ErlDrvSizeT from int. Check all code that use that field.

The ErlDrvSysInfo structure is used for storage of information about the Erlang runtime system. driver_system_info writes the system information when passed a reference to a ErlDrvSysInfo structure. The fields in the structure are as follows:

The ErlDrvBinary structure is a binary, as sent between the emulator and the driver. All binaries are reference counted; when driver_binary_free is called, the reference count is decremented, when it reaches zero, the binary is deallocated. orig_size is the binary size and orig_bytes is the buffer. ErlDrvBinary has not a fixed size, its size is orig_size + 2 * sizeof(int).

Some driver calls, such as driver_enq_binary, increment the driver reference count, and others, such as driver_deq decrement it.

Using a driver binary instead of a normal buffer is often faster, as the emulator needs not to copy the data, only the pointer is used.

A driver binary allocated in the driver, with driver_alloc_binary, is to be freed in the driver (unless otherwise stated) with driver_free_binary. (Notice that this does not necessarily deallocate it, if the driver is still referred in the emulator, the ref-count will not go to zero.)

Driver binaries are used in the driver_output2 and driver_outputv calls, and in the queue. Also the driver callback outputv uses driver binaries.

If the driver for some reason wants to keep a driver binary around, for example in a static variable, the reference count is to be incremented, and the binary can later be freed in the stop callback, with driver_free_binary.

Notice that as a driver binary is shared by the driver and the emulator. A binary received from the emulator or sent to the emulator must not be changed by the driver.

Since ERTS 5.5 (Erlang/OTP R11B), orig_bytes is guaranteed to be properly aligned for storage of an array of doubles (usually 8-byte aligned).

ErlDrvData

A handle to driver-specific data, passed to the driver callbacks. It is a pointer, and is most often type cast to a specific pointer in the driver.

SysIOVec

A system I/O vector, as used by writev on Unix and WSASend on Win32. It is used in ErlIOVec.

The I/O vector used by the emulator and drivers is a list of binaries, with a SysIOVec pointing to the buffers of the binaries. It is used in driver_outputv and the outputv driver callback. Also, the driver queue is an ErlIOVec.

ErlDrvMonitor

When a driver creates a monitor for a process, a ErlDrvMonitor is filled in. This is an opaque data type that can be assigned to, but not compared without using the supplied compare function (that is, it behaves like a struct).

The driver writer is to provide the memory for storing the monitor when calling driver_monitor_process. The address of the data is not stored outside of the driver, so ErlDrvMonitor can be used as any other data, it can be copied, moved in memory, forgotten, and so on.

ErlDrvNowData

The ErlDrvNowData structure holds a time stamp consisting of three values measured from some arbitrary point in the past. The three structure members are:

megasecs

The number of whole megaseconds elapsed since the arbitrary point in time

secs

The number of whole seconds elapsed since the arbitrary point in time

microsecs

The number of whole microseconds elapsed since the arbitrary point in time

ErlDrvPDL

If certain port-specific data must be accessed from other threads than those calling the driver callbacks, a port data lock can be used to synchronize the operations on the data. Currently, the only port-specific data that the emulator associates with the port data lock is the driver queue.

Normally a driver instance has no port data lock. If the driver instance wants to use a port data lock, it must create the port data lock by calling driver_pdl_create.

Note

Once the port data lock has been created, every access to data associated with the port data lock must be done while the port data lock is locked. The port data lock is locked and unlocked by driver_pdl_lock, and driver_pdl_unlock, respectively.

A port data lock is reference counted, and when the reference count reaches zero, it is destroyed. The emulator at least increments the reference count once when the lock is created and decrements it once the port associated with the lock terminates. The emulator also increments the reference count when an async job is enqueued and decrements it when an async job has been invoked. Also, the driver is responsible for ensuring that the reference count does not reach zero before the last use of the lock by the driver has been made. The reference count can be read, incremented, and decremented by driver_pdl_get_refc, driver_pdl_inc_refc, and driver_pdl_dec_refc, respectively.

Read/write lock. Used to allow multiple threads to read shared data while only allowing one thread to write the same data. Multiple threads can read lock an rwlock at the same time, while only one thread can read/write lock an rwlock at a time.

Exports

void add_driver_entry(ErlDrvEntry *de)

Adds a driver entry to the list of drivers known by Erlang. The init function of parameter de is called.

Note

To use this function for adding drivers residing in dynamically loaded code is dangerous. If the driver code for the added driver resides in the same dynamically loaded module (that is, .so file) as a normal dynamically loaded driver (loaded with the erl_ddll interface), the caller is to call driver_lock_driver before adding driver entries.

Use of this function is generally deprecated.

void *driver_alloc(ErlDrvSizeT size)

Allocates a memory block of the size specified in size, and returns it. This fails only on out of memory, in which case NULL is returned. (This is most often a wrapper for malloc).

Memory allocated must be explicitly freed with a corresponding call to driver_free (unless otherwise stated).

This function is thread-safe.

ErlDrvBinary *driver_alloc_binary(ErlDrvSizeT size)

Allocates a driver binary with a memory block of at least size bytes, and returns a pointer to it, or NULL on failure (out of memory). When a driver binary has been sent to the emulator, it must not be changed. Every allocated binary is to be freed by a corresponding call to driver_free_binary (unless otherwise stated).

Notice that a driver binary has an internal reference counter. This means that calling driver_free_binary, it may not actually dispose of it. If it is sent to the emulator, it can be referenced there.

The driver binary has a field, orig_bytes, which marks the start of the data in the binary.

Performs an asynchronous call. The function async_invoke is invoked in a thread separate from the emulator thread. This enables the driver to perform time-consuming, blocking operations without blocking the emulator.

The async thread pool size can be set with command-line argument +A in erl(1). If an async thread pool is unavailable, the call is made synchronously in the thread calling driver_async. The current number of async threads in the async thread pool can be retrieved through driver_system_info.

If a thread pool is available, a thread is used. If argument key is NULL, the threads from the pool are used in a round-robin way, each call to driver_async uses the next thread in the pool. With argument key set, this behavior is changed. The two same values of *key always get the same thread.

To ensure that a driver instance always uses the same thread, the following call can be used:

If a thread is already working, the calls are queued up and executed in order. Using the same thread for each driver instance ensures that the calls are made in sequence.

The async_data is the argument to the functions async_invoke and async_free. It is typically a pointer to a structure containing a pipe or event that can be used to signal that the async operation completed. The data is to be freed in async_free.

When the async operation is done, ready_async driver entry function is called. If ready_async is NULL in the driver entry, the async_free function is called instead.

The return value is -1 if the driver_async call fails.

Note

As from ERTS 5.5.4.3 the default stack size for threads in the async-thread pool is 16 kilowords, that is, 64 kilobyte on 32-bit architectures. This small default size has been chosen because the amount of async-threads can be quite large. The default stack size is enough for drivers delivered with Erlang/OTP, but is possibly not sufficiently large for other dynamically linked-in drivers that use the driver_async functionality. A suggested stack size for threads in the async-thread pool can be configured through command-line argument +a in erl(1).

unsigned int driver_async_port_key(ErlDrvPort port)

Calculates a key for later use in driver_async. The keys are evenly distributed so that a fair mapping between port IDs and async thread IDs is achieved.

Note

Before Erlang/OTP R16, the port ID could be used as a key with proper casting, but after the rewrite of the port subsystem, this is no longer the case. With this function, you can achieve the same distribution based on port IDs as before Erlang/OTP R16.

long driver_binary_dec_refc(ErlDrvBinary *bin)

Decrements the reference count on bin and returns the reference count reached after the decrement.

This function is thread-safe.

Note

The reference count of driver binary is normally to be decremented by calling driver_free_binary.

driver_binary_dec_refc does not free the binary if the reference count reaches zero. Only use driver_binary_dec_refc when you are sure not to reach a reference count of zero.

long driver_binary_get_refc(ErlDrvBinary *bin)

Returns the current reference count on bin.

This function is thread-safe.

long driver_binary_inc_refc(ErlDrvBinary *bin)

Increments the reference count on bin and returns the reference count reached after the increment.

This function is thread-safe.

ErlDrvTermData driver_caller(ErlDrvPort port)

Returns the process ID of the process that made the current call to the driver. The process ID can be used with driver_send_term to send back data to the caller. driver_caller only returns valid data when currently executing in one of the following driver callbacks:

The caller of driver_create_port is allowed to manipulate the newly created port when driver_create_port has returned. When port level locking is used, the creating port is only allowed to manipulate the newly created port until the current driver callback, which was called by the emulator, returns.

Returns 0 if a monitor was removed and > 0 if the monitor no longer exists.

ErlDrvSizeT driver_deq(ErlDrvPort port, ErlDrvSizeT size)

Dequeues data by moving the head pointer forward in the driver queue by size bytes. The data in the queue is deallocated.

Returns the number of bytes remaining in the queue on success, otherwise -1.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

int driver_enq(ErlDrvPort port, char* buf, ErlDrvSizeT len)

Enqueues data in the driver queue. The data in buf is copied (len bytes) and placed at the end of the driver queue. The driver queue is normally used in a FIFO way.

The driver queue is available to queue output from the emulator to the driver (data from the driver to the emulator is queued by the emulator in normal Erlang message queues). This can be useful if the driver must wait for slow devices, and so on, and wants to yield back to the emulator. The driver queue is implemented as an ErlIOVec.

When the queue contains data, the driver does not close until the queue is empty.

The return value is 0.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

Enqueues a driver binary in the driver queue. The data in bin at offset with length len is placed at the end of the queue. This function is most often faster than driver_enq, because no data must be copied.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

The return value is 0.

int driver_enqv(ErlDrvPort port, ErlIOVec *ev, ErlDrvSizeT skip)

Enqueues the data in ev, skipping the first skip bytes of it, at the end of the driver queue. It is faster than driver_enq, because no data must be copied.

The return value is 0.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

Signals to Erlang that the driver has encountered an error and is to be closed. The port is closed and the tuple {'EXIT', error, Err} is sent to the port owner process, where error is an error atom (driver_failure_atom and driver_failure_posix) or an integer (driver_failure).

The driver is to fail only when in severe error situations, when the driver cannot possibly keep open, for example, buffer allocation gets out of memory. For normal errors it is more appropriate to send error codes with driver_output.

The return value is 0.

int driver_failure_eof(ErlDrvPort port)

Signals to Erlang that the driver has encountered an EOF and is to be closed, unless the port was opened with option eof, in which case eof is sent to the port. Otherwise the port is closed and an 'EXIT' message is sent to the port owner process.

The return value is 0.

void driver_free(void *ptr)

Frees the memory pointed to by ptr. The memory is to have been allocated with driver_alloc. All allocated memory is to be deallocated, only once. There is no garbage collection in drivers.

This function is thread-safe.

void driver_free_binary(ErlDrvBinary *bin)

Frees a driver binary bin, allocated previously with driver_alloc_binary. As binaries in Erlang are reference counted, the binary can still be around.

Starts monitoring a process from a driver. When a process is monitored, a process exit results in a call to the provided process_exit callback in the ErlDrvEntry structure. The ErlDrvMonitor structure is filled in, for later removal or compare.

Sends data from an I/O vector, ev, to the port owner process. It has a header buffer (hbuf and hlen), just like driver_output2.

Parameter skip is a number of bytes to skip of the ev vector from the head.

You get vectors of ErlIOVec type from the driver queue (see below), and the outputv driver entry function. You can also make them yourself, if you want to send several ErlDrvBinary buffers at once. Often it is faster to use driver_output or .

For example, if hlen is 2 and ev points to an array of three binaries, the port owner process receives [H1, H2, <<B1>>, <<B2>> | <<B3>>].

The return value is 0 for normal use.

The comment for driver_output_binary also applies for driver_outputv.

ErlDrvPDL driver_pdl_create(ErlDrvPort port)

Creates a port data lock associated with the port.

Note

Once a port data lock has been created, it must be locked during all operations on the driver queue of the port.

Returns a newly created port data lock on success, otherwise NULL. The function fails if port is invalid or if a port data lock already has been associated with the port.

long driver_pdl_dec_refc(ErlDrvPDL pdl)

Decrements the reference count of the port data lock passed as argument (pdl).

The current reference count after the decrement has been performed is returned.

This function is thread-safe.

long driver_pdl_get_refc(ErlDrvPDL pdl)

Returns the current reference count of the port data lock passed as argument (pdl).

This function is thread-safe.

long driver_pdl_inc_refc(ErlDrvPDL pdl)

Increments the reference count of the port data lock passed as argument (pdl).

The current reference count after the increment has been performed is returned.

This function is thread-safe.

void driver_pdl_lock(ErlDrvPDL pdl)

Locks the port data lock passed as argument (pdl).

This function is thread-safe.

void driver_pdl_unlock(ErlDrvPDL pdl)

Unlocks the port data lock passed as argument (pdl).

This function is thread-safe.

SysIOVec *driver_peekq(ErlDrvPort port, int *vlen)

Retrieves the driver queue as a pointer to an array of SysIOVecs. It also returns the number of elements in vlen. This is one of two ways to get data out of the queue.

Nothing is removed from the queue by this function, that must be done with driver_deq.

The returned array is suitable to use with the Unix system call writev.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

ErlDrvSizeT driver_peekqv(ErlDrvPort port, ErlIOVec *ev)

Retrieves the driver queue into a supplied ErlIOVecev. It also returns the queue size. This is one of two ways to get data out of the queue.

If ev is NULL, all ones that is -1 type cast to ErlDrvSizeT are returned.

Nothing is removed from the queue by this function, that must be done with driver_deq.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

int driver_pushq(ErlDrvPort port, char* buf, ErlDrvSizeT len)

Puts data at the head of the driver queue. The data in buf is copied (len bytes) and placed at the beginning of the queue.

The return value is 0.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

Puts data in the binary bin, at offset with length len at the head of the driver queue. It is most often faster than driver_pushq, because no data must be copied.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

The return value is 0.

int driver_pushqv(ErlDrvPort port, ErlIOVec *ev, ErlDrvSizeT skip)

Puts the data in ev, skipping the first skip bytes of it, at the head of the driver queue. It is faster than driver_pushq, because no data must be copied.

The return value is 0.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

int driver_read_timer(ErlDrvPort port, unsigned long *time_left)

Reads the current time of a timer, and places the result in time_left. This is the time in milliseconds, before the time-out occurs.

The return value is 0.

void *driver_realloc(void *ptr, ErlDrvSizeT size)

Resizes a memory block, either in place, or by allocating a new block, copying the data, and freeing the old block. A pointer is returned to the reallocated memory. On failure (out of memory), NULL is returned. (This is most often a wrapper for realloc.)

This function is used by drivers to provide the emulator with events to check for. This enables the emulator to call the driver when something has occurred asynchronously.

Parameter event identifies an OS-specific event object. On Unix systems, the functions select/poll are used. The event object must be a socket or pipe (or other object that select/poll can use). On Windows, the Win32 API function WaitForMultipleObjects is used. This places other restrictions on the event object; see the Win32 SDK documentation.

Parameter on is to be 1 for setting events and 0 for clearing them.

Parameter mode is a bitwise OR combination of ERL_DRV_READ, ERL_DRV_WRITE, and ERL_DRV_USE. The first two specify whether to wait for read events and/or write events. A fired read event calls ready_input and a fired write event calls ready_output.

Note

Some OS (Windows) do not differentiate between read and write events. The callback for a fired event then only depends on the value of mode.

ERL_DRV_USE specifies if we are using the event object or if we want to close it. On an emulator with SMP support, it is not safe to clear all events and then close the event object after driver_select has returned. Another thread can still be using the event object internally. To safely close an event object, call driver_select with ERL_DRV_USE and on==0, which clears all events and then either calls stop_select or schedules it to be called when it is safe to close the event object. ERL_DRV_USE is to be set together with the first event for an event object. It is harmless to set ERL_DRV_USE even if it already has been done. Clearing all events but keeping ERL_DRV_USE set indicates that we are using the event object and probably will set events for it again.

Note

ERL_DRV_USE was added in Erlang/OTP R13. Old drivers still work as before, but it is recommended to update them to use ERL_DRV_USE and stop_select to ensure that event objects are closed in a safe way.

The return value is 0, unless ready_input/ready_output is NULL, in which case it is -1.

This function is only thread-safe when the emulator with SMP support is used.

int driver_set_timer(ErlDrvPort port, unsigned long time)

Sets a timer on the driver, which will count down and call the driver when it is timed out. Parameter time is the time in milliseconds before the timer expires.

When the timer reaches 0 and expires, the driver entry function timeout is called.

Notice that only one timer exists on each driver instance; setting a new timer replaces an older one.

Return value is 0, unless the timeout driver function is NULL, in which case it is -1.

ErlDrvSizeT driver_sizeq(ErlDrvPort port)

Returns the number of bytes currently in the driver queue.

This function can be called from any thread if a port data lock associated with the port is locked by the calling thread during the call.

void driver_system_info(ErlDrvSysInfo *sys_info_ptr, size_t size)

Writes information about the Erlang runtime system into the ErlDrvSysInfo structure referred to by the first argument. The second argument is to be the size of the ErlDrvSysInfo structure, that is, sizeof(ErlDrvSysInfo).

Collects several segments of data, referenced by ev, by copying them in order to the buffer buf, of the size len.

If the data is to be sent from the driver to the port owner process, it is faster to use driver_outputv.

The return value is the space left in the buffer, that is, if ev contains less than len bytes it is the difference, and if ev contains len bytes or more, it is 0. This is faster if there is more than one header byte, as the binary syntax can construct integers directly from the binary.

Sets and gets limits that will be used for controlling the busy state of the port message queue.

The port message queue is set into a busy state when the amount of command data queued on the message queue reaches the high limit. The port message queue is set into a not busy state when the amount of command data queued on the message queue falls below the low limit. Command data is in this context data passed to the port using either Port ! {Owner, {command, Data}} or port_command/[2,3]. Notice that these limits only concerns command data that have not yet reached the port. The busy port feature can be used for data that has reached the port.

Valid limits are values in the range [ERL_DRV_BUSY_MSGQ_LIM_MIN, ERL_DRV_BUSY_MSGQ_LIM_MAX]. Limits are automatically adjusted to be sane. That is, the system adjusts values so that the low limit used is lower than or equal to the high limit used. By default the high limit is 8 kB and the low limit is 4 kB.

By passing a pointer to an integer variable containing the value ERL_DRV_BUSY_MSGQ_READ_ONLY, the currently used limit is read and written back to the integer variable. A new limit can be set by passing a pointer to an integer variable containing a valid limit. The passed value is written to the internal limit. The internal limit is then adjusted. After this the adjusted limit is written back to the integer variable from which the new value was read. Values are in bytes.

The busy message queue feature can be disabled either by setting the ERL_DRV_FLAG_NO_BUSY_MSGQdriver flag in the driver_entry used by the driver, or by calling this function with ERL_DRV_BUSY_MSGQ_DISABLED as a limit (either low or high). When this feature has been disabled, it cannot be enabled again. When reading the limits, both are ERL_DRV_BUSY_MSGQ_DISABLED if this feature has been disabled.

Processes sending command data to the port are suspended if either the port is busy or if the port message queue is busy. Suspended processes are resumed when neither the port or the port message queue is busy.

char *erl_drv_cond_name(ErlDrvCond *cnd)

void erl_drv_cond_signal(ErlDrvCond *cnd)

Signals on a condition variable. That is, if other threads are waiting on the condition variable being signaled, one of them is woken.

cnd is a pointer to a condition variable to signal on.

This function is thread-safe.

void erl_drv_cond_wait(ErlDrvCond *cnd, ErlDrvMutex *mtx)

Waits on a condition variable. The calling thread is blocked until another thread wakes it by signaling or broadcasting on the condition variable. Before the calling thread is blocked, it unlocks the mutex passed as argument. When the calling thread is woken, it locks the same mutex before returning. That is, the mutex currently must be locked by the calling thread when calling this function.

cnd is a pointer to a condition variable to wait on. mtx is a pointer to a mutex to unlock while waiting.

Note

erl_drv_cond_wait can return even if no one has signaled or broadcast on the condition variable. Code calling erl_drv_cond_wait is always to be prepared for erl_drv_cond_wait returning even if the condition that the thread was waiting for has not occurred. That is, when returning from erl_drv_cond_wait, always check if the condition has occurred, and if not call erl_drv_cond_wait again.

This function is thread-safe.

int erl_drv_consume_timeslice(ErlDrvPort port, int percent)

Gives the runtime system a hint about how much CPU time the current driver callback call has consumed since the last hint, or since the the start of the callback if no previous hint has been given.

port

Port handle of the executing port.

percent

Approximate consumed fraction of a full time-slice in percent.

The time is specified as a fraction, in percent, of a full time-slice that a port is allowed to execute before it is to surrender the CPU to other runnable ports or processes. Valid range is [1, 100]. The scheduling time-slice is not an exact entity, but can usually be approximated to about 1 millisecond.

Notice that it is up to the runtime system to determine if and how to use this information. Implementations on some platforms can use other means to determine the consumed fraction of the time-slice. Lengthy driver callbacks should, regardless of this, frequently call this function to determine if it is allowed to continue execution or not.

This function returns a non-zero value if the time-slice has been exhausted, and zero if the callback is allowed to continue execution. If a non-zero value is returned, the driver callback is to return as soon as possible in order for the port to be able to yield.

This function is provided to better support co-operative scheduling, improve system responsiveness, and to make it easier to prevent misbehaviors of the VM because of a port monopolizing a scheduler thread. It can be used when dividing lengthy work into some repeated driver callback calls, without the need to use threads.

See also the important warning text at the beginning of this manual page.

int erl_drv_equal_tids(ErlDrvTid tid1, ErlDrvTid tid2)

Compares two thread identifiers, tid1 and tid2, for equality.

Returns 0 it they are not equal, and a value not equal to 0 if they are equal.

Note

A thread identifier can be reused very quickly after a thread has terminated. Therefore, if a thread corresponding to one of the involved thread identifiers has terminated since the thread identifier was saved, the result of erl_drv_equal_tids does possibly not give the expected result.

This function is thread-safe.

int erl_drv_getenv(const char *key, char *value, size_t *value_size)

Retrieves the value of an environment variable.

key

A NULL-terminated string containing the name of the environment variable.

value

A pointer to an output buffer.

value_size

A pointer to an integer. The integer is used both for passing input and output sizes (see below).

When this function is called, *value_size is to contain the size of the value buffer.

On success, 0 is returned, the value of the environment variable has been written to the value buffer, and *value_size contains the string length (excluding the terminating NULL character) of the value written to the value buffer.

On failure, that is, no such environment variable was found, a value < 0 is returned. When the size of the value buffer is too small, a value > 0 is returned and *value_size has been set to the buffer size needed.

Warning

Do not use libc's getenv or similar C library interfaces from a driver.

This function is thread-safe.

void erl_drv_init_ack(ErlDrvPort port, ErlDrvData res)

Acknowledges the start of the port.

port

The port handle of the port (driver instance) doing the acknowledgment.

res

The result of the port initialization. Can be the same values as the return value of start, that is, any of the error codes or the ErlDrvData that is to be used for this port.

When this function is called the initiating erlang:open_port call is returned as if the start function had just been called. It can only be used when flag ERL_DRV_FLAG_USE_INIT_ACK has been set on the linked-in driver.

Sends data in the special driver term format to the port owner process. This is a fast way to deliver term data from a driver. It needs no binary conversion, so the port owner process receives data as normal Erlang terms. The erl_drv_send_term functions can be used for sending to any process on the local node.

Note

Parameter port is not an ordinary port handle, but a port handle converted using driver_mk_port.

Parameter term points to an array of ErlDrvTermData with n elements. This array contains terms described in the driver term format. Every term consists of 1-4 elements in the array. The first term has a term type and then arguments. Parameter port specifies the sending port.

Tuples, maps, and lists (except strings, see below) are built in reverse polish notation, so that to build a tuple, the elements are specified first, and then the tuple term, with a count. Likewise for lists and maps.

A tuple must be specified with the number of elements. (The elements precede the ERL_DRV_TUPLE term.)

A map must be specified with the number of key-value pairs N. The key-value pairs must precede the ERL_DRV_MAP in this order: key1,value1,key2,value2,...,keyN,valueN. Duplicate keys are not allowed.

A list must be specified with the number of elements, including the tail, which is the last term preceding ERL_DRV_LIST.

The special term ERL_DRV_STRING_CONS is used to "splice" in a string in a list, a string specified this way is not a list in itself, but the elements are elements of the surrounding list.

The unsigned integer data type ErlDrvUInt and the signed integer data type ErlDrvSInt are 64 bits wide on a 64-bit runtime system and 32 bits wide on a 32-bit runtime system. They were introduced in ERTS 5.6 and replaced some of the int arguments in the list above.

The unsigned integer data type ErlDrvUInt64 and the signed integer data type ErlDrvSInt64 are always 64 bits wide. They were introduced in ERTS 5.7.4.

To build the tuple {tcp, Port, [100 | Binary]}, the following call can be made.

Here bin is a driver binary of length at least 50 and drvport is a port handle. Notice that ERL_DRV_LIST comes after the elements of the list, likewise ERL_DRV_TUPLE.

The ERL_DRV_STRING_CONS term is a way to construct strings. It works differently from how ERL_DRV_STRING works. ERL_DRV_STRING_CONS builds a string list in reverse order (as opposed to how ERL_DRV_LIST works), concatenating the strings added to a list. The tail must be specified before ERL_DRV_STRING_CONS.

ERL_DRV_STRING constructs a string, and ends it. (So it is the same as ERL_DRV_NIL followed by ERL_DRV_STRING_CONS.)

The ERL_DRV_EXT2TERM term type is used for passing a term encoded with the external format, that is, a term that has been encoded by erlang:term_to_binary, erl_interface:ei(3), and so on. For example, if binp is a pointer to an ErlDrvBinary that contains term {17, 4711} encoded with the external format, and you want to wrap it in a two-tuple with the tag my_tag, that is, {my_tag, {17, 4711}}, you can do as follows:

If you want to pass a binary and do not already have the content of the binary in an ErlDrvBinary, you can benefit from using ERL_DRV_BUF2BINARY instead of creating an ErlDrvBinary through driver_alloc_binary and then pass the binary through ERL_DRV_BINARY. The runtime system often allocates binaries smarter if ERL_DRV_BUF2BINARY is used. However, if the content of the binary to pass already resides in an ErlDrvBinary, it is normally better to pass the binary using ERL_DRV_BINARY and the ErlDrvBinary in question.

The ERL_DRV_UINT, ERL_DRV_BUF2BINARY, and ERL_DRV_EXT2TERM term types were introduced in ERTS 5.6.

This function is only thread-safe when the emulator with SMP support is used.

int erl_drv_putenv(const char *key, char *value)

Sets the value of an environment variable.

key is a NULL-terminated string containing the name of the environment variable.

value is a NULL-terminated string containing the new value of the environment variable.

Returns 0 on success, otherwise a value != 0.

Note

The result of passing the empty string ("") as a value is platform-dependent. On some platforms the variable value is set to the empty string, on others the environment variable is removed.

Warning

Do not use libc's putenv or similar C library interfaces from a driver.

This function is thread-safe.

ErlDrvRWLock *erl_drv_rwlock_create(char *name)

Creates an rwlock and returns a pointer to it.

name is a string identifying the created rwlock. It is used to identify the rwlock in planned future debug functionality.

Returns NULL on failure. The driver creating the rwlock is responsible for destroying it before the driver is unloaded.

This function is thread-safe.

void erl_drv_rwlock_destroy(ErlDrvRWLock *rwlck)

Destroys an rwlock previously created by erl_drv_rwlock_create. The rwlock must be in an unlocked state before it is destroyed.

rwlck is a pointer to an rwlock to destroy.

This function is thread-safe.

char *erl_drv_rwlock_name(ErlDrvRWLock *rwlck)

Returns a pointer to the name of the rwlock.

rwlck is a pointer to an initialized rwlock.

Note

This function is intended for debugging purposes only.

void erl_drv_rwlock_rlock(ErlDrvRWLock *rwlck)

Read locks an rwlock. The calling thread is blocked until the rwlock has been read locked. A thread that currently has read or read/write locked the rwlock cannot lock the same rwlock again.

rwlck is a pointer to the rwlock to read lock.

Warning

If you leave an rwlock locked in an emulator thread when you let the thread out of your control, you will very likely deadlock the whole emulator.

This function is thread-safe.

void erl_drv_rwlock_runlock(ErlDrvRWLock *rwlck)

Read unlocks an rwlock. The rwlock currently must be read locked by the calling thread.

rwlck is a pointer to an rwlock to read unlock.

This function is thread-safe.

void erl_drv_rwlock_rwlock(ErlDrvRWLock *rwlck)

Read/write locks an rwlock. The calling thread is blocked until the rwlock has been read/write locked. A thread that currently has read or read/write locked the rwlock cannot lock the same rwlock again.

rwlck is a pointer to an rwlock to read/write lock.

Warning

If you leave an rwlock locked in an emulator thread when you let the thread out of your control, you will very likely deadlock the whole emulator.

This function is thread-safe.

void erl_drv_rwlock_rwunlock(ErlDrvRWLock *rwlck)

Read/write unlocks an rwlock. The rwlock currently must be read/write locked by the calling thread.

rwlck is a pointer to an rwlock to read/write unlock.

This function is thread-safe.

int erl_drv_rwlock_tryrlock(ErlDrvRWLock *rwlck)

Tries to read lock an rwlock.

rwlck is a pointer to an rwlock to try to read lock.

Returns 0 on success, otherwise EBUSY. A thread that currently has read or read/write locked the rwlock cannot try to lock the same rwlock again.

Warning

If you leave an rwlock locked in an emulator thread when you let the thread out of your control, you will very likely deadlock the whole emulator.

This function is thread-safe.

int erl_drv_rwlock_tryrwlock(ErlDrvRWLock *rwlck)

Tries to read/write lock an rwlock. A thread that currently has read or read/write locked the rwlock cannot try to lock the same rwlock again.

rwlckis pointer to an rwlock to try to read/write lock.

Returns 0 on success, otherwise EBUSY.

Warning

If you leave an rwlock locked in an emulator thread when you let the thread out of your control, you will very likely deadlock the whole emulator.

A string identifying the created thread. It is used to identify the thread in planned future debug functionality.

tid

A pointer to a thread identifier variable.

func

A pointer to a function to execute in the created thread.

arg

A pointer to argument to the func function.

opts

A pointer to thread options to use or NULL.

Returns 0 on success, otherwise an errno value is returned to indicate the error. The newly created thread begins executing in the function pointed to by func, and func is passed arg as argument. When erl_drv_thread_create returns, the thread identifier of the newly created thread is available in *tid. opts can be either a NULL pointer, or a pointer to an ErlDrvThreadOpts structure. If opts is a NULL pointer, default options are used, otherwise the passed options are used.

The created thread terminates either when func returns or if erl_drv_thread_exit is called by the thread. The exit value of the thread is either returned from func or passed as argument to erl_drv_thread_exit. The driver creating the thread is responsible for joining the thread, through erl_drv_thread_join, before the driver is unloaded. "Detached" threads cannot be created, that is, threads that do not need to be joined.

Warning

All created threads must be joined by the driver before it is unloaded. If the driver fails to join all threads created before it is unloaded, the runtime system most likely crashes when the driver code is unloaded.

This function is thread-safe.

void erl_drv_thread_exit(void *exit_value)

Terminates the calling thread with the exit value passed as argument. exit_value is a pointer to an exit value or NULL.

int erl_drv_thread_join(ErlDrvTid tid, void **exit_value)

Joins the calling thread with another thread, that is, the calling thread is blocked until the thread identified by tid has terminated.

tid is the thread identifier of the thread to join. exit_value is a pointer to a pointer to an exit value, or NULL.

Returns 0 on success, otherwise an errno value is returned to indicate the error.

A thread can only be joined once. The behavior of joining more than once is undefined, an emulator crash is likely. If exit_value == NULL, the exit value of the terminated thread is ignored, otherwise the exit value of the terminated thread is stored at *exit_value.

This function is thread-safe.

char *erl_drv_thread_name(ErlDrvTid tid)

Returns a pointer to the name of the thread.

tid is a thread identifier.

Note

This function is intended for debugging purposes only.

ErlDrvThreadOpts *erl_drv_thread_opts_create(char *name)

Allocates and initializes a thread option structure.

name is a string identifying the created thread options. It is used to identify the thread options in planned future debug functionality.

Returns NULL on failure. A thread option structure is used for passing options to erl_drv_thread_create. If the structure is not modified before it is passed to erl_drv_thread_create, the default values are used.

Warning

You are not allowed to allocate the ErlDrvThreadOpts structure by yourself. It must be allocated and initialized by erl_drv_thread_opts_create.

void *erl_drv_tsd_get(ErlDrvTSDKey key)

Returns the thread-specific data associated with key for the calling thread.

key is a thread-specific data key.

Returns NULL if no data has been associated with key for the calling thread.

This function is thread-safe.

int erl_drv_tsd_key_create(char *name, ErlDrvTSDKey *key)

Creates a thread-specific data key.

name is a string identifying the created key. It is used to identify the key in planned future debug functionality.

key is a pointer to a thread-specific data key variable.

Returns 0 on success, otherwise an errno value is returned to indicate the error. The driver creating the key is responsible for destroying it before the driver is unloaded.

This function is thread-safe.

void erl_drv_tsd_key_destroy(ErlDrvTSDKey key)

Destroys a thread-specific data key previously created by erl_drv_tsd_key_create. All thread-specific data using this key in all threads must be cleared (see erl_drv_tsd_set) before the call to erl_drv_tsd_key_destroy.

key is a thread-specific data key to destroy.

Warning

A destroyed key is very likely to be reused soon. Therefore, if you fail to clear the thread-specific data using this key in a thread before destroying the key, you will very likely get unexpected errors in other parts of the system.

This function is thread-safe.

void erl_drv_tsd_set(ErlDrvTSDKey key, void *data)

Sets thread-specific data associated with key for the calling thread. You are only allowed to set thread-specific data for threads while they are fully under your control. For example, if you set thread-specific data in a thread calling a driver callback function, it must be cleared, that is, set to NULL, before returning from the driver callback function.

key is a thread-specific data key.

data is a pointer to data to associate with key in the calling thread.

Warning

If you fail to clear thread-specific data in an emulator thread before letting it out of your control, you might never be able to clear this data with later unexpected errors in other parts of the system as a result.

This function is thread-safe.

char *erl_errno_id(int error)

Returns the atom name of the Erlang error, given the error number in error. The error atoms are einval, enoent, and so on. It can be used to make error terms from the driver.

int remove_driver_entry(ErlDrvEntry *de)

Driver entries added by the erl_ddll Erlang interface cannot be removed by using this interface.

void set_busy_port(ErlDrvPort port, int on)

Sets and unsets the busy state of the port. If on is non-zero, the port is set to busy. If it is zero, the port is set to not busy. You typically want to combine this feature with the busy port message queue functionality.

Processes sending command data to the port are suspended if either the port or the port message queue is busy. Suspended processes are resumed when neither the port or the port message queue is busy. Command data is in this context data passed to the port using either Port ! {Owner, {command, Data}} or port_command/[2,3].